Craniocervical posture in patients with skeletal malocclusion and its correlation with craniofacial morphology during different growth periods

The association between craniocervical posture and craniofacial structures in the various sagittal skeletal malocclusion during different growth stages has been the focus of intense interest in fields of orthodontics, but it has not been conclusively demonstrated. Thus, this study aimed to investigate the association between craniofacial morphology and craniocervical posture in patients with sagittal skeletal malocclusion during different growth periods. A total of 150 from a large pool of cephalograms qualified for the inclusion and exclusion were evaluated and classified into three groups according to the Cervical Vertebral Maturation (CVM) by examining the morphological modifications of the second through fourth cervical vertebrae, each group consisted of 50 cephalograms. In each growth period, for the comparison of head and cervical posture differences among various skeletal classes, the radiographs were further subdivided into skeletal Class I (0° < ANB < 5°, n = 16), skeletal Class II (ANB ≥ 5°, n = 18), and skeletal Class III (0° ≤ ANB, n = 16) on the basis of their ANB angle. There was no significant difference in gender (P > 0.05). Some variables were found to be significant during pubertal growth and later in patients with sagittal skeletal malocclusion (P < 0.05). Most indicators describing craniocervical posture were largest in skeletal Class II and smallest in skeletal Class III during the peak growth periods and later. Cervical inclination variables were greater in skeletal Class III than in skeletal Class II. Variables of craniofacial morphology and craniocervical posture are more correlated during the pubertal growth period and later in patients with sagittal skeletal malocclusion. A tendency is an indication of the close interrelationship that a more extended head was in skeletal Class II while a flexed head was in skeletal Class III. Nevertheless, with the considerations of some limitations involved in this study, further longitudinal studies with large samples are required to elucidate the relationship clearly.


Sample design and setting
This study is a cross-sectional study, the test level α = 0.05 was set up, the test efficacy of 1 − β = 0.09, the standard deviation σ is expected to be 4 through literature, the allowed error δ is 0.75, the Z (1 − α/2) = 1.96.According to the formula n = (Z 1−α/2 *σ/δ) 2 , the minimum total sample size was calculated (n = 109).Finally, a sample of 150 individuals were included in the present study.To reduce heterogeneity brought about by gender differences, men and women were divided equally.The study was conducted with the pretreatment lateral cephalometric radiographs, a total of 225 patients who had attended the department of orthodontics for seeking treatment were randomly selected from the record archives.75 subjects were not included according to the inclusion and exclusion criteria.Finally, a total of 150 (75 females and 75 males, aged 7-18 years) were selected on the basis of the following inclusion criteria.The study was reviewed and approved by the local ethics committee.Informed consent was obtained from the subjects and/or their parents.

Inclusion and exclusion criteria
Subjects were included if they had: (1) Sagittal skeletal malocclusion; (2) Lateral cephalometric radiographs taken in the natural head position (NHP) and at least four clear cervical spine shapes; (3) A Chinese ethnic origin; (4)  No history of orthodontic treatment or orthognathic surgery.Subjects were excluded if they had: (1) Deleterious oral habits such as oral breathing or chewing side preference; (2) A history of severe vertical or horizontal development collapse; (3) Neurological and respiratory diseases; (4) Potential craniomaxillofacial disorders such as temporomandibular joint disorders or cleft lip and palate; (5) Family history.

Lateral cephalometric radiographs
Cephalograms were routinely obtained with ProMax (Planmeca, Helsinki, Finland) in the NHP.Exposure was operated at 80 kV, 10 mv.The NHP of the patient was determined by positioning the subjects in a standing, selfbalanced position in which they felt comfortable and relaxed.Evidence 37,38 has shown excellent stability for 5 or www.nature.com/scientificreports/even 15 years after the initial radiograph.Nine craniofacial morphology-associated variables of sagittal skeletal malocclusion and nine variables representing craniocervical, craniovertical, and cervicohorizontal and cervical curvature angles 16,30,32,[39][40][41] were measured using Myorthox measurement tool.
The cephalograms were classified into three groups according to the CVM method 42 , CS12 group (the prepeak stage, n = 50), CS34 group (the peak growth, n = 50), and CS56 group (the post-peak stage, n = 50), representing different growth and development stages 43 with an equal distribution of men and women by the two investigators (HP and LY).These radiographs were further subdivided into skeletal Class I (0° < ANB < 5°, n = 16, normal facial pattern without sagittal skeletal discrepancy), skeletal Class II (ANB ≥ 5°, n = 18, a convex profile showing a marked protruded maxilla or retruded mandible or a combination of both), and skeletal Class III (0° ≤ ANB, n = 16, a concave profile showing marked maxillary retrusion or mandibular protrusion or a combination of both) based on the ANB angle in each growth period, the profiles of skeletal class II and skeletal class III malocclusions are shown in Fig. S1, the flow chart of the sample stratification is shown in the Fig. 1.

Variables and data measurement
Variables associated with craniofacial morphology such as SNA, SNB, ANB, FH/ML, NSL/NL, NSL/ML, NA-PA, NP-FH, Y axis in patients with sagittal skeletal malocclusion were measured on the cephalograms.Craniocervical posture is mainly determined by the angle formed by Odontoid Process Tangent (OPT) and Nasion-Sella line (NSL) (NSL/OPT), the angle between Nasal line (NL) and OPT (NL/OPT), the angle between NSL and Cervical Vertebra Tangent (CVT) (NSL/CVT), and the angle between NL and CVT (NL/CVT).Cervical inclination was assessed using cervicohorizontal angles, such as the angle formed by OPT and horizontal line (HOR) (OPT/ HOR), and the angle between CVT and HOR (CVT/HOR).Meanwhile, head position was represented using the craniovertical angles, such as the angle formed by NSL and VER (NSL/VER), and the angle between NL and VER (NL/VER).Cervical curvature was determined by measuring the angle formed by OPT and CVT (OPT/ CVT).The definitions of reference points and planes are shown in Table S1, reference variables are described in Table 1, detailed landmarks and measurement items on the Cephalograms are shown in Fig. 2.

Measurement error
Analysis of internal consistency, the intraclass correlation coefficients (ICC) were calculated for both intra-and inter-examiner concordance.An ICC value higher than 0.75 are indicative of good reliability 45 .This involved comparing the CVM groupings by two different reviewers (HP and LY) as well as analyzing the measurements taken by the same investigator (HP) after a 2-week interval.

Statistical analysis
Descriptive variables were conducted using the Statistical Package for Social Sciences (SPSS), version 26.0 (SPSS Inc, Chicago, IL, USA).The quantitative postural variables were calculated as the mean (M) and standard deviation (SD).Pearson's analysis was used to evaluate the correlation between craniocervical posture and craniofacial morphology in patients with sagittal skeletal malocclusion during different growth periods, and assess the potential impact of growth by comparing the correlation coefficients in various growth periods.An independent sample t-test was employed to investigate the potential influence of gender on the correlation between craniocervical posture and craniofacial morphology in sagittal skeletal malocclusion.A one-way analysis of variances (ANOVA) was used to examine and compare the intergroup differences in cervical posture across different skeletal classes during the same growth period.Significance was set at p ≤ 0.05.

Ethical approval and informed consent
The study was conducted in accordance with the Declaration of Helsinki, and approved by the Ethics Committee of College of Stomatology, Chongqing Medical University (NO: 2022 LSNo.088) and Chongqing University

Impact of growth period on the relationship between craniofacial morphology and craniocervical posture in sagittal skeletal malocclusion
Correlation analysis of variables at the pre-peak growth period ANB was positively correlated with most variables, NSL/VER, NL/VER, NSL/OPT, NL/OPT, NSL/CVT, NL/ CVT, and inversely correlated with OPT/HOR, CVT/HOR, OPT/CVT.A mild positive correlation coefficient of 0.285 was observed between ANB and NSL/VER, while a negative correlation coefficient of − 0.299 was observed between ANB and OPT/HOR.Only these two parameters exhibit statistical significances (P < 0.05).SNA and SNB was negatively correlated with NSL/CVT (P < 0.05) and SNB was inversely correlated with NSL/OPT as well (P < 0.05).NSL/NL was negatively correlated with NL/VER, NL/OPT, NL/CVT (P < 0.05).There were positive correlations between NSL/ML, FH/ML, and the Y-axis with NSL/VER, NSL/ML was positively correlated with NSL/CVT (P < 0.05).In contrast, NP/FH was negatively correlated with NSL/VER (P < 0.05).NA/PA has no significant correlation with any of the head and cervical posture variables.However, the correlation coefficients were low in all variables (Table 3).

Reference variables Description Characterization
Craniofacial morphology variables

SNA Sella-Nasion-A angle
The prognathism of the maxilla to the cranial base 32

SNB Sella-Nasion-B angle
The prognathism of the mandible to the cranial base 32 ANB A-Nasion-B angle Difference between SNA and SNB, which determines anterioposterior relationship of the maxillary and mandibular bases 32,40 FH/ML Angle between FH and ML Frankfort mandibular plane angle 30 NSL/ML Angle between NSL and ML Mandibular plane angle 30 NSL/OPT Angle between NSL and OPT 30,44 Craniocevical angles NSL/CVT Angle between NSL and CVT 30,44 NL/OPT Angle between NL and OPT 30,44 NL/CVT Angle between NL and CVT 30,44 CVT/HOR Angle between CVT and HOR 30,44 Cervicohorizontal angles OPT/HOR Angle between OPT and HOR 30,44 NSL/VER Angle between NSL and VER 30,44 Craniovertical angles NL/VER Angle between NL and VER 30,44 OPT/CVT Angle between odontoid process tangent and cervical vertebra tangent 30,44 Cervical curvature angle   All parameters describing craniocervical angles (NSL/OPT, NL/OPT, NSL/CVT, NL/CVT) were largest in the skeletal Class II and lowest in the skeletal Class III during the peak and post-peak periods, showing statistical significances in the peak periods and NL/CVT variable during the post-peak periods (P < 0.05).The above most variables were largest in skeletal Class I and smallest in skeletal Class III during the pre-peak growth period, but only NL/OPT variable showed a significance (P < 0.05) (Table 6).The inclination of the cervical spine (OPT/ HOR, and CVT/HOR) was greater in skeletal Class III than skeletal Class II in the overall growth periods, but no statistical significances were shown (P > 0.05).
The largest cervical curvature was exhibited in skeletal Class III than skeletal Class II in the pre-peak growth period, which is contrary to the results obtained in the peak and post-peak growth periods, exhibiting the

Discussion
Under normal physiological conditions, the stability of cervical curvature is not achieved until the age of seven 46 .Previous studies 47,48 have underscored the significance of attached muscles, especially the sternocleidomastoid muscle 49,50 , in providing stability and supporting the posture of the head and neck.Studies have proposed that the convergence of sensory information at the trigeminal nuclei from various structures such as the periodontium, masticatory muscles, jaws, and cervical spine region can influence the neuroanatomical connections between posture and the stomatognathic system 47,51,52 .Thus, adjustments in head and cervical posture are required to attain a personally perceived comfortable position, which compensates for alterations in stomatognathic homeostasis 53 .Moreover, the head and neck posture gradually reorient themselves towards a normal direction through gravity modifications acting on the craniocervical structures 54 .In a study conducted by Kondo et al. 55 , it demonstrated that an interdisciplinary approach involving early occlusal improvement and physiotherapy to establish a harmonious balance between the neck and masticatory muscles was effective in enhancing both facial appearance and posture.
In the present study, no significant gender difference was found in the correlation of craniofacial morphology with craniocervical posture in sagittal skeletal pattern.It can be inferred that these results are independent of gender, which are in agreement with some of the findings in previous studies 25,30 .Although not all groups exhibited statistically significant differences, it's worth noting that there are more correlated between most craniofacial morphology variables and craniocervical posture during the peak and post-peak growth periods, which indicated an interrelation between craniofacial morphology and craniocervical posture in the development of sagittal skeletal discrepancies.Furthermore, the correlation between malocclusion and craniocervical condition appears to change with growth.These results suggest that maxilla components may have a less significant impact on determining craniocervical posture compared to the mandibular.As opposed to the maxilla, the results indicate a strong relationship to mandibular-related indicators at the peak growth periods, significant differences were observed in the majority of parameters related to the mandible and craniocervical posture, suggesting that the size and position of the mandible are two primary factors strongly associated with head and cervical posture, which is in line with previous studies [56][57][58][59] .A significantly higher extension of the head upon the spine was observed.Therefore, these results underscored the significance of clinicians in assisting adolescent patients seeking orthodontic treatment, clinicians should pay attention to the maturity of the cervical spine as indicated in lateral cephalograms, as well as to identify and address any detrimental postural habits to prevent the development of poor posture and malocclusion.
Cervical curvature (OPT/CVT) is an essential determinant for evaluating the posture of the cervical spine.The measurement of OPT/CVT was greatest in the skeletal Class II and lowest in the skeletal Class III both during the growth spurt period and in subsequent stages.The values of OPT/HOR and CVT/HOR were found to be highest in individuals with skeletal Class III malocclusion across all growth stages, demonstrating a tendency that the cervical spine inclined more dorsally in skeletal Class III malocclusion, while the cervical column tilted more forward in skeletal Class II malocclusion, although this difference was not statistically significant, this trend is consistent with other studies 30,33 .These findings suggest a potential relationship between craniofacial morphology and craniocervical posture in patients with sagittal skeletal malocclusion during the pubertal growth period, which may provide valuable insights into the assessment of head and cervical position in the field of orthodontics.Hence, it would be beneficial for future studies to concentrate on adolescents during or after their peak growth period and to further investigate the association between the cervical spine and various craniomaxillofacial indicators within this stage.This could be a key population for preventing the development of this condition and developing a more comprehensive and interdisciplinary treatment approach.
Although the changes in variables are not stable and these could have an impact on the results of the current study, which may be related to the dynamic growth process that children experience, the current findings display a tendency that the cervical spine was more inclined forward in skeletal Class II and flexed in skeletal Class III, and is strongly supported by other studies 30,32 .The craniomaxillofacial region and cervical spine are susceptible to environmental factors, such as mouth breathing, heavy load, significantly impacting on body posture, particularly during the growth spurt stage 60,61 .Therefore, it also emphasized the need to address any postural changes as early as possible, preferably within the younger age group.This early intervention aims to mitigate growth deformations, minimize the power investment by the body, and optimize mechanical efficiency 62 .These findings could aid the orientation of future research though the results did not provide us with well-support conclusive results.Taking the effect of natural changes in growing patients into consideration, the present study investigated the relationship between craniofacial morphology with the craniocervical position based on the maturity of second through fourth cervical vertebrae, reflecting a more precise age estimation in individuals with sagittal skeletal discrepancies.To accurately evaluate the growth stage of an individual, compared with chronological age, skeletal age 63 is considered the optimal determinant, which can reflect individual growth and maturity more accurately and efficiently by examinations of CVM and hand-wrist x-ray mostly.Evidences have demonstrated that the assessment of CVM was comparable to hand-wrist analysis in terms of determining skeletal age.Furthermore, lateral cephalometric radiographs are routinely obtained for clinical orthodontic practice avoiding additional x-ray exposure [64][65][66][67] .
However, there remain several limitations in the present study.Therefore.it is necessary to emphasize that the caution should be applied in interpreting and promoting findings presented in this study.Firstly, considering that most skeletal class I patients have dental malocclusion problems such as crowding 68 , anterior open bite 69 , deep overjet 70 , and these factors should also be taken into consideration.Due to ethical considerations, for the www.nature.com/scientificreports/absence of control group and its limitations as a cross-sectional study regarding growth evaluation, which is insufficient sensitivity to individual variability.Thus, it is recommended to conduct more studies that evaluate a group of subjects in a longitudinal manner with a multi-center approach to address the limitations of the current study design and enhance the understanding of this topic.Secondly, it is important to note that many of these measurements still remain standardization and validation as reliable tools for postural assessment.Therefore, a comprehensive range of parameters should be employed to accurately depict the craniofacial structures associated with deviated craniocervical posture.A study 11 suggested the utilization of geometric morphometric methods, is a more effective approach compared to conventional cephalometric analysis for visually evaluating variations in the morphology and size among different skeletal classes.It was found that the curve fitting method is a suitable approach for assessing cervical curvature, although its application in clinical practice is challenging due to the requirement of C7 vertebra tracing.D' Attilio et al. 32 demonstrated that the lower part of the spine was straighter in skeletal Class III subjects compared to those with skeletal Class I and Class II subjects.It is evident that using the angle formed by the OPT and CVT lines alone to describe cervical column curvature is insufficient.Further investigations are needed to identify more relevant indicators of cervical inclination.Additionally, a more comprehensive approach for appraisal and evaluation of the lower part of the spinal column should be taken into account in future studies.
In fact, head and cervical posture appeared to be associated with both the sagittal facial dimension and the vertical development of the face.These patterns have important implications for clinical diagnosis and treatment prognosis in practice, thus, vertical facial patterns should be taken into account in further research.However, other limitation of the study is that the results did not reveal a cause-effect relationship behind the interactions between occlusion and posture.Therefore, it is crucial to conduct more longitudinal studies with a higher level of evidence and extended follow-up periods to further investigate the relationship.However, it is crucial to highlight the significance of assessing craniocervical posture as an integral part of the clinical practice, as it may play a vital role in facilitating a comprehensive orthodontic diagnosis and treatment planning.

Conclusion
Variables of craniofacial morphology and craniocervical posture are more correlated during the pubertal growth period and later in patients with sagittal skeletal pattern and a more extended head was displayed in the skeletal Class II relationship while a flexed head was exhibited in the skeletal Class III relationship, These findings highlight significant changes in head and cervical posture during periods of rapid growth, which is important to provide clinical decisions for orthodontic treatment and prognosis in the dynamic process of growth.However, taking the limitations of the study into account, further more prospective studies with large samples, good study designs are needed to clearly elucidate this association.

Figure 1 .
Figure 1.The flow chart of the sample stratification.
NSL/NL Angle between NSL and NL The maxilla to cranial base NA-PA Angle between NA and PA Reflection of the protrusion of the maxilla NP-FH Angle between NP and FH Reflection of the projection of the mandibular Y axis Angle between SGn and FH Reflection of projection of the chin and the direction of facial growth Head and cervical postural variables

Table 2 .
Gender differences among variables.

Table 4 .
Correlation analysis at the peak growth period.

comparison of head and neck postural variables among different skeletal classes within each growth period
Indicators describing the head posture, NSL/VER and NL/VER, were largest in skeletal Class II and smallest in skeletal Class III during the peak and post-peak periods, NSL/VER and NL/VER during the peak growth period, NL/VER during the post-peak growth period have shown statistically significances (P < 0.05).NSL/VER and NL/ VER was greater in Class II than in Class III at the pre-peak growth period, showing no statistically significances (P > 0.05) (Tables 6, 7 and 8).

Table 8 .
Differencesmallest in skeletal Class III and the largest in skeletal Class II, though these differences were not statistically significant (p > 0.05).However, corresponding results will require further confirmed.